Differential charge-balancing during high-frequency neural stimulation

ABSTRACT

Differential charge-balancing can be used in high-frequency neural stimulation. For example, a neural stimulation apparatus can have first and second electrodes configured to be coupled proximate to a nerve fiber to implement a neural stimulation procedure. A neural stimulation circuit can be electrically coupled to the first and second electrodes. The neural stimulation circuit can apply stimulation currents to the nerve fiber through the first and second electrodes during a first stimulation phase of the neural stimulation procedure. The neural stimulation circuit can also apply a modified stimulation current to the nerve fiber through the first electrode during a second stimulation phase of the neural stimulation procedure. The modified stimulation current can be generated based on a difference between (i) a voltage at the first electrode, and (ii) a reference voltage derived from voltages on the first and second electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 16/566,025 filed Sep. 10, 2019, which claims priority to ProvisionalApplication No. 62/729,479, filed on Sep. 11, 2018, the entirety ofwhich are hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates generally to electrical therapeuticsystems. More specifically, but not by way of limitation, thisdisclosure relates differential charge-balancing during high-frequencyneural stimulation.

BACKGROUND

Many chronic diseases, such as epilepsy and depression, can be treatedby stimulating the nerves in a patient's body using electrical signals.While such neural stimulation procedures are typically implemented usingelectrical signals with lower frequencies, e.g., 20 hertz (Hz) to 1 kHz,recent studies show that some diseases may be treatable using electricalsignals with higher frequencies, e.g., from 1 kHz to 50 kHz.

SUMMARY

One example of the present disclosure includes a system comprising a setof electrodes configured to be coupled to a nerve fiber to implement aneural stimulation procedure. The system also comprises a stimulationcircuit electrically coupled to the set of electrodes. The stimulationcircuit is configured to apply stimulation currents through the set ofelectrodes to the nerve fiber during a stimulation phase of a treatmentcycle in the neural stimulation procedure. The stimulation circuit isalso configured to apply recovery currents through the set of electrodesto the nerve fibers during a recovery phase that is subsequent to thestimulation phase in the treatment cycle, the recovery currents havingopposite polarities to the respective stimulation currents. Thestimulation circuit is also configured to adjust one or morecharacteristics of the stimulation currents or the recovery currentsbased on a reference voltage derived from voltages on the set ofelectrodes, the one or more characteristics of the stimulation currentsor the recovery currents being adjusted to reduce a charge buildup onthe set of electrodes resulting at least partially from the stimulationphase and the recovery phase. The stimulation circuit is also configuredto apply the stimulation currents or the recovery currents with theadjusted one or more characteristics through the set of electrodes tothe nerve fiber during a subsequent treatment cycle of the neuralstimulation procedure to reduce the charge buildup.

Another example of the present disclosure includes a method comprisingapplying stimulation currents through a set of electrodes to a nervefiber during a stimulation phase of a treatment cycle in a neuralstimulation procedure. The method also comprises applying recoverycurrents through the set of electrodes to the nerve fibers during arecovery phase that is subsequent to the stimulation phase in thetreatment cycle, the recovery currents having opposite polarities to therespective stimulation currents. The method also comprises adjusting oneor more characteristics of the stimulation currents or the recoverycurrents based on a reference voltage derived from voltages on the setof electrodes, the one or more characteristics of the stimulationcurrents or the recovery currents being adjusted to reduce a chargebuildup on the set of electrodes resulting at least partially from thestimulation phase and the recovery phase. The method also comprisesapplying the stimulation currents or the recovery currents with theadjusted one or more characteristics through the set of electrodes tothe nerve fiber during a subsequent treatment cycle of the neuralstimulation procedure to reduce the charge buildup. Some or all of thesesteps can be performed by a neural stimulation device.

Still another example of the present disclosure includes a neuralstimulation apparatus comprising a first electrode and a secondelectrode configured to be coupled proximate to a nerve fiber toimplement a neural stimulation procedure. The neural stimulationapparatus also comprises a neural stimulation circuit electricallycoupled to the first electrode and the second electrode. The neuralstimulation circuit is configured to apply stimulation currents to thenerve fiber through the first electrode and the second electrode duringa first stimulation phase of the neural stimulation procedure. Theneural stimulation circuit is also configured to apply a modifiedstimulation current to the nerve fiber through the first electrodeduring a second stimulation phase of the neural stimulation procedure,wherein the modified stimulation current is generated based on adifference between (i) a voltage at the first electrode, and (ii) areference voltage derived from voltages on the first electrode and thesecond electrode (e.g., as a result of the first stimulation phase).

Yet another example of the present disclosure includes a methodcomprising applying stimulation currents to a nerve fiber through afirst electrode and a second electrode during a first stimulation phaseof a neural stimulation procedure. The method also comprises applying amodified stimulation current to the nerve fiber through the firstelectrode during a second stimulation phase of the neural stimulationprocedure, wherein the modified stimulation current is generated basedon a difference between (i) a voltage at the first electrode, and (ii) areference voltage derived from voltages on the first electrode and thesecond electrode (e.g., as a result of the first stimulation phase).Some or all of these steps can be performed by a neural stimulationapparatus.

These illustrative examples are mentioned not to limit or define thescope of this disclosure, but rather to provide examples to aidunderstanding thereof. Illustrative examples are discussed in theDetailed Description, which provides further description. Advantagesoffered by various examples may be further understood by examining thisspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an example of a neural stimulation deviceaccording to some aspects of the present disclosure.

FIG. 2 is a circuit diagram showing a more detailed example of theneural stimulation device of FIG. 1 according to some aspects.

FIG. 3 is a circuit diagram showing an example of the neural stimulationdevice of FIG. 2 during the stimulation and recovery phases of a neuralstimulation procedure according to some aspects.

FIG. 4 is a circuit diagram showing an example of the neural stimulationdevice of FIG. 2 during a sampling phase of a neural stimulationprocedure according to some aspects.

FIG. 5 is a circuit diagram showing another example of the neuralstimulation device of FIG. 2 during a sampling phase of a neuralstimulation procedure according to some aspects.

FIG. 6 is a graph of an example of voltages across electrodes over thecourse of several treatment cycles during a neural stimulation procedureaccording to some aspects.

FIG. 7 is a flow chart of an example of a process for implementing aneural stimulation procedure according to some aspects.

FIG. 8 is a block diagram of an example of the upper half of a neuralstimulation device according to some aspects.

DETAILED DESCRIPTION

Reference will now be made in detail to various and alternativeillustrative examples and to the accompanying drawings. Each example isprovided by way of explanation and not as a limitation. It will beapparent to those skilled in the art that modifications and variationsmay be made. For instance, features illustrated or described as part ofone example may be used in another example to yield a still furtherexample. Thus, it is intended that this disclosure includesmodifications and variations as come within the scope of the appendedclaims and their equivalents.

Illustrative Example of Providing Differential Charge-Balancing DuringNeural Stimulation

One illustrative example of the present disclosure includes a neuralstimulation device for applying high-frequency neural stimulation to apatient. The neural stimulation device includes a neural stimulationcircuit coupled to a first electrode and a second electrode. Theelectrodes can be coupled to a nerve fiber in the patient's body, suchas a nerve fiber in the patient's arm. The neural stimulation device canuse the neural stimulation circuit to repeatedly apply current to thenerve fiber (via the electrodes) in a series treatment cycles, whichcollectively form a neural stimulation procedure.

Each of the treatment cycles can include a series of phases (e.g.,stages). In the illustrative example, the phases include a stimulationphase and a recovery phase. During the stimulation phase, the neuralstimulation device can apply a stimulation current having a certainpolarity through the electrodes to the nerve fiber in order to stimulatethe nerve. But repeatedly applying the stimulation current to the nervefiber can cause charge to build up on the electrodes, which canpotentially damage the nerve fiber. So, the neural stimulation devicecan next implement a recovery phase to reduce the charge build-up on theelectrodes. During the recovery phase, the neural stimulation device canapply a recovery current through the electrodes to the nerve fiber. Therecovery current can have a similar waveform but an opposite polarity tothe stimulation current. The goal of the recovery phase is to entirelyoffset the charge build-up on the electrodes from the stimulation phaseand return them to their normal state, but the recovery current is oftennot a perfect inverse replica of the stimulation current due topractical inefficiencies. As a result, a residual charge is often leftover on the electrodes after the recovery phase. And if the stimulationand recovery phases are quickly repeated during several treatment cyclesof a high-frequency neural stimulation procedure, the residual charge onthe electrodes can rapidly build up to dangerous levels (e.g., greaterthan 0.5 v) at which chemical reactions can occur that damage the nervefibers and the surrounding tissue.

To reduce or eliminate these dangers, in the illustrative example, theneural stimulation device can also implement a sampling phase. Thesampling phase can occur after the recovery phase in each treatmentcycle of the neural stimulation procedure. During the sampling phase,the neural stimulation device can derive a reference voltage from thevoltages on the first and second electrodes. Specifically, the residualcharge resulting from the stimulation and recovery phases can manifestas different voltages on the different electrodes. The neuralstimulation device can derive a reference value from those voltages.Examples of the reference voltage can include an average of thevoltages, a combination of the voltages, an aggregation of the voltages,a difference between the voltages, etc. After deriving the referencevoltage, in some examples, the neural stimulation device can use thereference voltage to adjust one or more characteristics (e.g., anamplitude, frequency, wave shape, and/or duration) of the stimulationcurrents applied to the electrodes during the next stimulation phase ofthe next treatment cycle. For example, the neural stimulation device caninclude a feedback loop in which the difference between each electrode'svoltage and the reference voltage is used to adjust the stimulationcurrent driving the electrode during the next stimulation phase of thenext treatment cycle. Additionally or alternatively, the neuralstimulation device can use the reference voltage to adjust one or morecharacteristics of the recovery currents applied to the electrodesduring the next recovery phase of the next treatment cycle. For example,the feedback loop can use the difference between each electrode'svoltage and the reference voltage to adjust the recovery current drivingthe electrode during the next recovery phase of the next treatmentcycle. In this way, the stimulation and/or recovery currents can beadjusted differentially during each treatment cycle to prevent theelectrodes from building up dangerous levels of residual charge over thecourse several treatment cycles, without reducing the total amount ofstimulation current and/or recovery current supplied to the patientduring the neural stimulation procedure.

While the above example involves two electrodes for simplicity, otherexamples can involve more electrodes. For instance, neural stimulationdevice can include five electrodes through which the neural stimulationdevice can apply five stimulation currents during the stimulation phaseand five recovery currents during the recovery phase. During thesampling phase, the neural stimulation device can derive a referencevoltage from the voltages on the five electrodes, and then use thedifference between each electrode's voltage and the reference voltage toadjust the stimulation current and/or recovery current driving theelectrodes during the next treatment cycle.

The description of the illustrative example above is provided merely asan example, not to limit or define the limits of the present subjectmatter. Various other examples are described herein and variations ofsuch examples would be understood by one of skill in the art. Advantagesoffered by various examples may be further understood by examining thisspecification and/or by practicing one or more examples of the claimedsubject matter.

Illustrative Systems and Methods for Providing DifferentialCharge-Balancing During High-Frequency Neural Stimulation

FIG. 1 is a circuit diagram of an example of a neural stimulationcircuit 100 according to some aspects of the present disclosure. Theneural stimulation circuit 100 can be used during a neural stimulationprocedure, such as neuromodulation therapy, to treat a chronic disease.During the neural stimulation procedure, current sources I1-I2 deliverelectrical current to a nerve fiber within a patient's tissue(represented by impedance R1 in FIG. 1 ) through electrodes E1-E2positioned on or within the patient's tissue.

As the current flows through the electrodes E1-E2 to the patient'stissue, a double-layer capacitance may form at the interfaces betweenthe electrodes E1-E2 and the tissue. For example, if the voltage acrosselectrode E1 and the tissue is small (e.g., less than 0.5 v), thencapacitance DLC1 is generated. And if the voltage across electrode E2and the tissue is small, then capacitance DLC2 is also generated,thereby yielding a double-layer capacitance formed from DLC1 and DLC2.But if the voltage across one or both of these interfaces is large(e.g., greater than 0.5 v), then undesirable chemical reactions canoccur that create a direct current (DC) path between the electrodesE1-E2, which can damage the tissue.

One way that the voltages across these interfaces can become dangerouslylarge is through charge buildup on the electrodes E1-E2. Charge buildupcan occur when the neural stimulation circuit 100 implements multipletreatment cycles of a neural stimulation procedure. Each treatment cyclecan involve a stimulation phase. During the stimulation phase, theneural stimulation circuit 100 can close switches SW1 and SW4, whileleaving switches SW2 and SW3 open. Examples of a switch can include atransistor, relay, or single-pole single-throw switch. With the switchesin the above-described configuration, current can then flow from thecurrent source I1 to electrode E1, through the tissue to electrode E2,and then to current source I4, as shown by the dashed line. Eachtreatment cycle can also include a recovery phase, which can occur afterthe stimulation phase. During the recovery phase, the neural stimulationcircuit 100 can close switches SW3 and SW2, while leaving switches SW1and SE4 open. Current can then flow from the current source I2 toelectrode E2, through the tissue to electrode E1, and then to currentsource I3, in the direction opposite to the dashed line. After thestimulation phase and the recovery phase, some residual charge typicallyremains on the electrodes E1-E2. If the residual charge is not reducedor eliminated, the voltages on the electrodes E1-E2 can build up todangerous levels after multiple treatment cycles.

To avoid these dangers, some examples of the present disclosure includea feedback circuit 102. The neural stimulation circuit 100 can activatethe feedback circuit 102 during a sampling phase that occurs after therecovery phase. The feedback circuit 102 is an active circuit that usesa reference voltage derived from voltages on the electrodes E1-E2 toadjust the stimulation currents and/or recovery currents applied by thecurrent sources I1-I2 during a subsequent treatment cycle. This isdescribed in greater detail below with respect to FIG. 2 .

FIG. 2 shows a more detailed example of the upper half of the neuralstimulation circuit 100 of FIG. 1 according to some aspects. As shown inFIG. 2 , the feedback circuit 102 can generally include two halvescorresponding to the two electrodes E1-E2. The left half can correspondto electrode E1 and can include switches SW5-SW9; capacitors Cs1, Cr1and Cb1; and voltage-to-current converter Gm1. The left half can feed anelectrical signal back to a transistor of the current source I1 toadjust stimulation and/or recovery currents provided by current sourceI1. The right half can correspond to electrode E2 and can includeswitches SW10-SE14; capacitors Cs2, Cr2, and Cb2; and voltage-to-currentconverter Gm2. The right half can feed an electrical signal back to atransistor of the current source I2 to adjust stimulation and/orrecovery currents provided by current source I2. The two halves meet inthe middle at common nodes 200 and 202. Operation of the neuralstimulation circuit 100 with the feedback circuit 102 is discussed belowwith respect to FIGS. 3-5 .

FIG. 3 shows an example of the upper half of the neural stimulationcircuit 100 during the stimulation and recovery phases of a neuralstimulation procedure. Although not shown in FIG. 3 for simplicity, acontrol element (e.g., one or more processors, timers, and/or crystals)can be coupled to any combination of the switches SW1-SW12. The controlelement can transmit control signals to the switches SW1-SW12 in orderto change their states and thereby implement the various phases of atreatment cycle. For example, the control element can initiate astimulation phase by transmitting a close signal (e.g., a high signal)configured to cause the switch SW1 to close and an open signal (e.g., alow signal) configured to cause the switch SW3 to open. When theswitches SW1, SW3 are in these states, a stimulation current can flowfrom the current source I1 through electrodes E1-E2, thereby stimulatingthe patient's tissue and generating a positive voltage across theelectrodes E1-E2. An example of this positive voltage during thestimulation phase is shown by line V_(E1-2) in the graph 300. Once thestimulation phase is complete, the control element can initiate arecovery phase by transmitting an open signal configured to cause theswitch SW1 to open and a close signal configured to cause the switch SW3to close. When the switches SW3, SW1 are in these states, a recoverycurrent can flow in the opposite direction from the current source I2through electrodes E1-E2, thereby generating a negative voltage acrossthe electrodes E1-E2. An example of this negative voltage during therecovery phase is shown by line V_(E1-2) in the graph 300. At the end ofthe recovery phase, a residual charge (AV) remains on the electrodes.

During the stimulation and recovery phases, the components of thefeedback circuit 102 shown using darker colors in FIG. 3 are active,while the components of the feedback circuit 102 shown in lighter colorsin FIG. 3 are inactive. For example, during both of these phases, thecontrol element can open switches SW5 and SW10 to prevent current flowto the feedback circuit 102. The control element can also close theswitches SW6 and SW11 to reset the sampling capacitors Cs1-Cs2.Additionally, the control element can open the switches SW8 and SW13 toprevent current flow from reference capacitors Cr1-Cr2 to the currentsources I1-I2, and close switches SW9 and SW14 to enable referencecapacitors Cr1-Cr2 to charge to the fixed voltage (Vf) at the commonnode 202.

After the recovery phase, the control element can initiate the samplingphase by transmitting open signals configured to cause some or all ofthe switches SW1-SW4 to open, for example, as shown in FIGS. 4-5 . Inthose figures, the components of the feedback circuit 102 shown usingdarker colors in FIGS. 4-5 are active, while the components of thefeedback circuit 102 shown in lighter colors in FIGS. 4-5 are inactive.

Referring now to FIG. 4 , during the sampling phase, the control elementcan transmit close signals to switches SW5, SW8, SW10, and SW13 to closethose switches. An example of such a close signal is represented byϕ_(samp) in graph 400. And the control element can also transmit opensignals to switches SW6, SW9, SW11, and SW14 to open those switches. Anexample of such an open signal is represented by φ _(samp) in graph 400.When the feedback circuit 102 is in this switch configuration, thesampling capacitors Cs1-Cs2 can generate the reference voltage (e.g., anaverage voltage) between the electrodes E1-E2 at the common node 200.The sampling capacitors Cs1-Cs2 can also each charge to a voltage levelthat is the difference between each electrode's voltage and thereference voltage at node 200. For example, sampling capacitor Cs1 cancharge to a voltage level that is the difference between the voltage atelectrode E1 and the reference voltage at the common node 200. Andsampling capacitor Cs2 can charge to a voltage level that is thedifference between the voltage at electrode E2 and the reference voltageat the common node 200. This process can be referred to as sampling,whereby the sampling capacitors Cs1-Cs2 are sampling the differencesbetween each electrode's voltage and the reference voltage at the commonnode 200.

During the sampling phase, the control element can further activate thevoltage-to-current converters Gm1-Gm2 for a brief time period, as shownin FIG. 5 . Referring to FIG. 5 , the control element can transmit closesignals to switches SW7 and SW12 to close those switches. An example ofsuch a close signal is represented by ϕ_(GmEn) in graph 500. With switchSW7 closed, the voltage-to-current converter Gm1 can generate a firstsignal (e.g., an output current) that is related to the voltage acrossthe sampling capacitor Cs1. Since the voltage across the samplingcapacitor Cs1 is the difference between the reference voltage at thecommon node 200 and the voltage at electrode E1, the first signal fromthe voltage-to-current converter Gm1 can be related to the differencebetween the reference voltage at the common node 200 and the voltage atelectrode E1. The voltage-to-current converter Gm1 can transmit thefirst signal toward a terminal (e.g., gate) of the current source I1, asshown by a dashed arrow in FIG. 5 . The first signal can combine withanother signal (e.g., a fixed current) from the reference capacitor Cr1to arrive at a combined signal that is fed back to the current sourceI1. Providing the combined signal to the current source I1 can cause thecurrent source I1 to adjust a characteristic of the stimulation currentapplied to electrode E1 during a subsequent stimulation phase and/or therecovery current applied to electrode E1 during a subsequent recoveryphase.

Similarly to the above, with switch SW12 closed, the voltage-to-currentconverter Gm2 can generate a second signal that is related to (e.g.,proportional to) the voltage across the sampling capacitor Cs2. Sincethe voltage across the sampling capacitor Cs2 is the difference betweenthe reference voltage at the common node 200 and the voltage atelectrode E2, the second signal from the voltage-to-current converterGm2 can be related to the difference between the reference voltage atthe common node 200 and the voltage at electrode E2. Thevoltage-to-current converter Gm2 can transmit the second signal toward aterminal of the current source I2. The second signal can combine withanother signal from the capacitor Cr2 to arrive at a combined signalthat is fed back to the current source I2. Providing the combined signalto the current source I2 can cause the current source I2 to adjust acharacteristic of the stimulation current applied to electrode E2 duringa subsequent stimulation phase and/or the recovery current applied toelectrode E2 during a subsequent recovery phase.

The adjustments to the stimulation and/or recovery currents may beslight in order to incrementally reduce the residual voltage on theelectrodes E1-E2 to zero over several treatment cycles. One example ofthis process is shown in FIG. 6 . FIG. 6 depicts eight treatment cycles,though any number of treatment cycles is possible. Each treatment cycleincludes a stimulation phase (ϕ₁), a recovery phase (ϕ₂), and a samplingphase (ϕ₃). Over the course of the treatment cycles, each of thesephases is applied periodically. As a result, the residual voltage onelectrodes E1-E2 is gradually adjusted such that it oscillates aroundand ultimately reduces to approximately zero.

While the examples shown in FIGS. 2-5 depict two electrodes withcorresponding feedback components, the neural stimulation circuit 100can include any number and combination of electrodes with correspondingcurrent sources and feedback components. As one such example, the neuralstimulation circuit 100 can include a third current source coupled to athird electrode (e.g., E3) to deliver stimulation and recovery currentsto the patient's tissue via the third electrode. The neural stimulationcircuit 100 can also include feedback components coupled between thethird current source and the third electrode, whereby the feedbackcomponents can be configured in any of the arrangements discussed above.For example, the feedback components can include a sampling capacitor(e.g., Cs3), a voltage-to-current converter (e.g., Gm3), a referencecapacitor (e.g., Cr3), another capacitor (e.g., Cb3), and variousswitches. These components can be configured as shown in FIGS. 2-5 ,with a lead of the sampling capacitor coupled to the common node 200 togenerate a reference voltage based on the voltages on electrodes E1-E3at the common node 200, and a lead of the reference capacitor can becoupled to the common node 202 for charging to the fixed voltage Vf.

Further, while the feedback circuit 102 shown in FIGS. 2-5 has a certainarrangement of circuit components, other arrangements of circuitcomponents are possible. Some examples can include more circuitcomponents, fewer circuit components, different circuit components, or adifferent combination of the circuit components than are shown in FIGS.2-5 . For instance, some examples may use various digital components(e.g., logic gates, processors, analog-to-digital converters, ordigital-to-analog converters) additionally or alternatively to theanalog components depicted in FIGS. 2-5 . In one such example, thefeedback circuit 102 can include a processor additionally oralternatively to the voltage-to-current converters Gm1-Gm2. Theprocessor can detect the voltage across the sampling capacitor Cs1 andresponsively transmit a first signal to the current source I1 to adjusta first stimulation current that it applies during a subsequentstimulation phase and/or a first recovery current that it applies duringa subsequent recovery phase. The processor can also detect the voltageacross the sampling capacitor Cs2 and responsively transmit a secondsignal to the current source I2 to adjust a second stimulation currentthat it applies during a subsequent stimulation phase and/or a secondrecovery current that it applies during a subsequent recovery phase.

As another example, the feedback circuit 102 can include comparatorsadditionally or alternatively to the voltage-to-current convertersGm1-Gm2 and/or the sampling capacitors Cs1-Cs2. The comparators caninclude a first comparator configured to compare the voltage on theelectrode E1 to the reference voltage to determine if voltage on theelectrode E1 is above or below the reference voltage. If the chargebuildup on the electrode E1 is greater than the reference voltage, thefirst comparator can output a first signal (e.g., a current pulse) tothe current source I1 to cause the current source I1 to adjust a firststimulation current that it applies during a subsequent stimulationphase and/or a first recovery current that it applies during asubsequent recovery phase. Likewise, the comparators can include asecond comparator configured to compare the voltage on the electrode E2to the reference voltage to determine if voltage on the electrode E2 isabove or below the reference voltage. If the charge buildup on theelectrode E2 is greater than the reference voltage, the secondcomparator can output a second signal to the current source I2 to causethe current source I2 to adjust a second stimulation current that itapplies during a subsequent stimulation phase and/or a second recoverycurrent that it applies during a subsequent recovery phase.

Yet another example that relies more heavily on digital circuitry isshown in FIG. 8 , which depicts the upper half of a neural stimulationcircuit 100 according to some aspects. The neural stimulation circuit100 includes the feedback circuit 102. The feedback circuit 102 candetect a voltage associated with the electrodes E1-E2 and control thecurrent sources I1-I2 to adjust the stimulation currents and/or therecovery currents that are subsequently applied.

More specifically, the feedback circuit 102 includes one or more sensors802 configured to detect a voltage associated with the electrodes E1-E2.Examples of the sensors 802 can include a voltmeter or a capacitor, suchas capacitor Cs1 of FIG. 2 . The sensors 802 may detect a voltage at E1,a voltage at E2, a reference voltage, or any combination of these. Thesensors 802 can output sensor signals indicative of the detectedvoltages. One or more quantizers 804 can receive the sensor signals fromthe sensors 802 and convert the voltages detected by the sensors 802into digital signals. An example of a quantizer can include ananalog-to-digital converter, such as a 5-bit analog-to-digitalconverter. The quantizers 804 can transmit the digital signals todigital-signal processing circuitry 806, which may include logic gates,such as AND, OR, NOT, and NAND gates; processors, such as proportionalintegral controllers; integrated circuit (IC) components, such asamplifiers or comparators; or any combination of these. Thedigital-signal processing circuitry 806 can receive the digital signalsand generate first and second signals based on the digital signals. Thedigital-signal processing circuitry 806 can then transmit the first andsecond signals to the current sources I1-I2, respectively, to controlthe current sources I1-I2 (e.g., to adjust the stimulation currentsand/or recovery currents that are subsequently applied).

In some examples, the quantizers 804 can include a quantizer that isdedicated to each electrode. So, if there are two electrodes (e.g.,electrodes E1-E2), there will be two quantizers. And if there are threeelectrodes, there will be three quantizers. And so on. Each quantizercan receive an analog signal from the sensors 802, where the analogsignal represents a voltage at the quantizer's corresponding electrode.Each quantizer can then generate a digital signal based on the analogsignal and transmit the digital signal to the digital-signal processingcircuitry 806, which in turn can process the digital signals todetermine how to adjust the stimulation currents and/or recoverycurrents applied by the current sources I1-I2.

In other examples, the quantizers 804 can include a multi-channelquantizer. The multi-channel quantizer may have as many channels asthere are electrodes. For example, the multi-channel quantizer caninclude a first channel corresponding to electrode E1 and a secondchannel corresponding to electrode E2. Each channel can receive ananalog signal representing a voltage detected at its correspondingelectrode. For example, the first channel can receive a first analogsignal representing a first voltage detected at electrode E1, and thesecond channel can receive a second analog signal representing a secondvoltage detected at electrode E2. The multi-channel quantizer cangenerate one or more digital signals based on the received analogsignals. For example, the multi-channel quantizer can generate a singledigital signal based on the first analog signal associated withelectrode E1 and the second analog signal associated with E2, where thesingle digital signal is representative of a difference between thefirst voltage at electrode E1 and the second voltage at electrode E2.The multi-channel quantizer can then transmit the digital signal(s) tothe digital-signal processing circuitry 806.

The digital-signal processing circuitry 806 can receive the digitalsignal(s) from the quantizer(s) 804 and control the current sourcesI1-I2 based on the digital signal(s). For example, digital-signalprocessing circuitry 806 can determine a voltage at E1, a voltage at E2,a reference voltage (e.g., a difference between the voltages at E1-E2),or any combination of these, based on the digital signal(s). Thedigital-signal processing circuitry 806 can then control the currentsources I1-I2 based on the determined voltage, so as to adjust thecharacteristics (e.g., amplitudes, pulse widths, or both) of thestimulation currents and/or recovery currents. This may reduce chargebuildup.

As one specific example, the digital-signal processing circuitry 806 caninclude a proportional integral controller. The proportional integralcontroller can control the current sources I1-I2 based on the digitalsignal(s) received from the quantizer(s) 804, so as to adjust thecharacteristics of stimulation currents and/or recovery currents. Forinstance, the proportional integral controller can generate a firstsignal configured to increase an amount of current output by the currentsource I1 by a certain amount (e.g., +10 mA), and generate a secondsignal configured to decrease an amount of current output by the currentsource I2 by the same amount (e.g., −10 mA). This may result in thecurrents output by the current sources I1-I2 generally offsetting eachother, which can reduce charge buildup.

While the example shown in FIG. 8 depicts two electrodes E1-E2, theneural stimulation circuit 100 can include any number and combination ofelectrodes with corresponding feedback components. Further, while thefeedback circuit 102 shown in FIG. 8 has a certain arrangement ofcomponents, other examples can include more components, fewercomponents, different components, or a different arrangement of thecomponents than are shown in FIG. 8 . For instance, some examples maylack separate sensors 802 and instead rely on the quantizers 804 toserve as the sensors (e.g., to detect the voltage), since a quantizercan generally measure voltage directly. And some examples may involvethe quantizers 804 being part of (e.g., internal to) a processor of thedigital-signal processing circuitry 806. Further, any aspects discussedabove in relation to FIG. 8 can be combined with any with aspectsdiscussed above with respect to FIGS. 1-5 to yield still furtherexamples.

FIG. 7 is a flow chart of an example of a process for implementing aneural stimulation procedure according to some aspects. Other examplescan include more steps, fewer steps, different steps, or a differentcombination of steps than are shown in FIG. 7 . The steps of FIG. 7 arediscussed below with reference to the components discussed above inrelation to FIGS. 2-5 .

In block 702, a neural stimulation device uses a neural stimulationcircuit 100 to implement a stimulation phase of a neural stimulationprocedure. More specifically, the neural stimulation device initiatesthe stimulation phase by operating a control element, which transmits aclose signal to switch SW1 and an open signal to switch SW3. The closesignal causes switch SW1 to close and the open signal causes switch SW3to open. Closing switch SW1 completes a circuit between the currentsource I1 and electrode E1, thereby enabling the current source I1 totransmit stimulation current to electrode E1, through a nerve fiber inthe patient's tissue to electrode E2, and then towards ground. Thestimulation current stimulates the nerve fiber in the patient's tissueas it passes between electrodes E1 and E2, but also leaves an electriccharge on the electrodes E1-E2 as a result.

The amount of stimulation current transmitted to the nerve fiber canchange depending on the treatment cycle. For example, during the firsttreatment cycle of the neural stimulation procedure, the current sourceI1 can be configured to transmit an initial amount of stimulationcurrent to the nerve fiber. The initial amount of stimulation currentcan be set by an operator of the neural stimulation device (e.g., aspart of the neural stimulation procedure). For example, the neuralstimulation device can receive user input specifying the initial amountof stimulation current to be delivered to the nerve fiber during thefirst treatment cycle. In response to the user input, the neuralstimulation device can configure the current source I1 to deliver thatamount of stimulation current to the nerve fiber during the firsttreatment cycle. During subsequent cycles of the neural stimulationprocedure, the amount of stimulation current delivered to the nervefiber during the stimulation phase can be adjusted by the feedbackcircuit 102, for example, as discussed in greater detail below.

In block 704, the neural stimulation device uses the neural stimulationcircuit 100 to implement a recovery phase of the neural stimulationprocedure. More specifically, the neural stimulation device can end thestimulation phase and initiate the recovery phase by operating thecontrol element such that the control element transmits an open signalto switch SW1 and a close signal to switch SW3. The open signal causesswitch SW1 to open and the close signal causes switch SW3 to close.Closing switch SW3 completes a circuit between the current source I2 andelectrode E2, thereby enabling the current source I2 to transmit arecovery current to electrode E2, through the nerve fiber in thepatient's tissue to electrode E1, and then towards ground. This recoverycurrent stimulates the nerve fiber as it passes between electrodes E1and E2. Since the recovery current can have an opposite polarity to thestimulation current, the recovery current can reduce the amount ofelectric charge on the electrodes E1-E2. However, there may still be aresidual charge leftover at the end of the recovery phase.

The amount of recovery current transmitted to the nerve fiber can changedepending on the treatment cycle. For example, during the firsttreatment cycle of the neural stimulation procedure, the current sourceI2 can be configured to transmit an initial amount of recovery currentto the nerve fiber. The initial amount of recovery current can be set byan operator of the neural stimulation device (e.g., as part of theneural stimulation procedure). For example, the neural stimulationdevice can receive user input specifying the initial amount of recoverycurrent to be delivered to the nerve fiber during the first treatmentcycle. In response to the user input, the neural stimulation device canconfigure the current source I2 to deliver that amount of recoverycurrent to the nerve fiber during the first treatment cycle. Duringsubsequent cycles of the neural stimulation procedure, the amount ofrecovery current delivered to the nerve fiber during the stimulationphase can be adjusted by the feedback circuit 102, for example, asdiscussed in greater detail below.

In block 706, the neural stimulation device uses the neural stimulationcircuit 100 to implement a sampling phase of the neural stimulationprocedure. More specifically, the neural stimulation device can end therecovery phase and initiate the sampling phase by operating the controlelement such that the control element transmits open signals to switchesSW1 and SW3. The open signals cause the switches SW1 and SW3 to open,preventing current flow between the current sources I1-I2 and theelectrodes E1-E2.

With switches SW1 and SW3 open, the control element can then implementthe sampling phase by operating the switches SW5-SW14 (e.g., asdiscussed above) to generate first and second signals based on areference voltage at common node 200. The feedback circuit 102 cansupply the first and second signals to the current sources I1-I2,respectively, thereby causing the current sources I1-I2 to adjustrespective characteristics of the stimulation currents in a subsequentstimulation phase and/or the recovery currents in a subsequent recoveryphase.

As a particular example, the control element can close switches SW5 andSW10. Closing these switches electrically couples the first electrode E1to the second electrode E2 at the common node 200, thereby producing thereference voltage at the common node 200. Closing these switches alsocauses the first sampling capacitor Cs1 to charge to a first voltage,whereby the first voltage represents a first voltage difference betweenthe first electrode's voltage and the reference voltage. Thevoltage-to-current converters Gm1 can then detect the first voltageacross the sampling capacitor Cs1 and generate the first signal based onthe first voltage across the first sampling capacitor Cs1. Likewise,closing switches SW5 and SW10 causes the second sampling capacitor Cs2to a second voltage, whereby the second voltage represents a secondvoltage difference between the second electrode's voltage and thereference voltage. The voltage-to-current converter Gm2 can detect thesecond voltage across the second sampling capacitor Cs2 and responsivelygenerate the second signal based on the second voltage across the secondsampling capacitor Cs2. The control element can then close switches SW7and SW12, enabling the voltage-to-current converters Gm1-Gm2 to transmitthe first and second signals towards the first and second currentsources I1-I2, respectively.

In some examples, the first and second signals are relativelyunadulterated before being fed back to the current sources I1-I2, whilein other examples each of the first and second signals can be mixed withother signals (e.g., from reference capacitors Cr1-Cr2) before being fedback to the current sources I1-I2. Either way, the current sources I1-I2can adjust one or more characteristics (e.g., an amplitude, frequency,waveform, and/or duration) of a stimulation current or a recoverycurrent applied during a subsequent treatment cycle based on the firstand second signals which, as discussed above, are themselves based onthe reference voltage derived from the voltages on the set of electrodesE1-E2. The one or more characteristics stimulation currents and/orrecovery currents can be adjusted to reduce a charge buildup on the setof electrodes E1-E2 resulting at least partially from the stimulationphase and the recovery phase.

In block 708, the neural stimulation device determines if the neuralstimulation procedure is complete. For example, the neural stimulationdevice may be preprogrammed to implement a neural stimulation procedurewith a particular number of treatment cycles, where each treatment cycleincludes some or all of the steps shown in blocks 702-706. The neuralstimulation procedure can use a counter to keep track of the number oftreatment cycles already implemented for determining whether there theneural stimulation procedure is complete. If the neural stimulationprocedure is complete, the process can end. Otherwise, the process canreturn to block 702 and iterate. For example, the process can return toblock 702 where the neural stimulation device can use the neuralstimulation circuit 100 to implement another stimulation phase, in whichthe neural stimulation circuit 100 applies stimulation currents and/orrecovery currents with the adjusted one or more characteristics (e.g.,as a result of the sampling phase in block 706) through the electrodesE1-E2 to the nerve fiber in order to reduce the charge buildup on theelectrodes E1-E2.

The foregoing description of certain examples, including illustratedexamples, has been presented only for the purpose of illustration anddescription and is not intended to be exhaustive or to limit thedisclosure to the precise forms disclosed. Numerous modifications,adaptations, and uses thereof will be apparent to those skilled in theart without departing from the scope of the disclosure. For instance,any example(s) described herein can be combined with any otherexample(s).

1. A method comprising: applying, by a current source of a neuralstimulation apparatus, a stimulation current to a nerve fiber through aset of electrodes during a first stimulation phase of a neuralstimulation procedure; and applying, by the current source of the neuralstimulation apparatus, a modified stimulation current to the nerve fiberthrough the set of electrodes during a second stimulation phase of theneural stimulation procedure, wherein the modified stimulation currentis generated based on a voltage difference between (i) a voltage at afirst electrode in the set of electrodes, and (ii) a reference voltagederived from voltages at the first electrode and a second electrode inthe set of electrodes.
 2. The method of claim 1, further comprisingapplying the stimulation current to the nerve fiber through a thirdelectrode during the first stimulation phase of the neural stimulationprocedure.
 3. The method of claim 2, wherein the reference voltage isderived from voltages at the first electrode, the second electrode, andthe third electrode.
 4. The method of claim 1, further comprisingadjusting one of more characteristics of the stimulation current togenerate the modified stimulation current by: generating a signal basedon the voltage difference; and supplying the signal to the currentsource, the current source being configured to receive the signal andadjust one or more characteristics of the stimulation current based onthe signal.
 5. The method of claim 4, wherein: the one or morecharacteristics include an amplitude, a wave shape, or a duration; andthe reference voltage is an average voltage among the set of electrodes.6. The method of claim 4, wherein the first electrode is electricallycoupled to the second electrode at a common node to produce thereference voltage at the common node, wherein a sampling capacitor iselectrically coupled between the common node and the first electrode,and further comprising: charging the sampling capacitor to a voltage,wherein the voltage represents the voltage difference between the firstelectrode's voltage and the reference voltage; and generating the signalbased on the voltage at the sampling capacitor.
 7. The method of claim6, wherein: the signal is generated by a voltage-to-current converter,the signal being a current that is proportional to the voltage acrossthe sampling capacitor.
 8. The method of claim 4, wherein: the currentsource includes a transistor with a gate; and supplying the signal tothe current source involves supplying the signal to the gate of thetransistor.
 9. The method of claim 4, wherein: the signal is generatedby a comparator configured to compare the voltage at the first electrodeto the reference voltage.
 10. A system comprising: a set of electrodesconfigured to be coupled to a nerve fiber to implement a neuralstimulation procedure; and a stimulation circuit electrically coupled tothe set of electrodes, the stimulation circuit being configured to:apply a stimulation current from a current source to the nerve fiberthrough the set of electrodes during a first stimulation phase of theneural stimulation procedure; and apply a modified stimulation currentfrom the current source to the nerve fiber through the set of electrodesduring a second stimulation phase of the neural stimulation procedure,wherein the modified stimulation current is generated based on a voltagedifference between (i) a voltage at a first electrode in the set ofelectrodes, and (ii) a reference voltage derived from voltages at thefirst electrode and a second electrode in the set of electrodes.
 11. Thesystem of claim 10, wherein the stimulation circuit is furtherconfigured to apply the stimulation current to the nerve fiber through athird electrode during the first stimulation phase of the neuralstimulation procedure.
 12. The system of claim 11, wherein the referencevoltage is derived from voltages at the first electrode, the secondelectrode, and the third electrode.
 13. The system of claim 10, whereinthe stimulation circuit is further configured to adjust one of morecharacteristics of the stimulation current to generate the modifiedstimulation current by: generating a signal based on the voltagedifference; and supplying the signal to the current source, the currentsource being configured to receive the signal and adjust one or morecharacteristics of the stimulation current based on the signal.
 14. Thesystem of claim 13, wherein: the one or more characteristics include anamplitude, a wave shape, or a duration; and the reference voltage is anaverage voltage among the set of electrodes.
 15. The system of claim 13,wherein the first electrode is electrically coupled to the secondelectrode at a common node to produce the reference voltage at thecommon node, wherein a sampling capacitor is electrically coupledbetween the common node and the first electrode, and wherein thestimulation circuit is further configured to: charge the samplingcapacitor to a voltage, wherein the voltage represents the voltagedifference between the first electrode's voltage and the referencevoltage; and generate the signal based on the voltage at the samplingcapacitor.
 16. The system of claim 15, further comprising avoltage-to-current converter that is configured to generate the signal,the signal being a current that is proportional to the voltage acrossthe sampling capacitor.
 17. The system of claim 13, wherein the currentsource includes a transistor with a gate, and wherein the stimulationcircuit is further configured to supply the signal to the current sourceby supplying the signal to the gate of the transistor.
 18. The system ofclaim 13, further comprising a comparator configured to generate thesignal based on a comparison of the voltage at the first electrode tothe reference voltage.
 19. A neural stimulation apparatus, comprising: acurrent source; a set of electrodes coupled to the current source, theset of electrodes being configured to be coupled to a nerve fiber toimplement a neural stimulation procedure; and a stimulation circuitelectrically coupled to the current source and the set of electrodes,the stimulation circuit being configured to: apply a stimulation currentfrom the current source to the nerve fiber through the set of electrodesduring a first stimulation phase of the neural stimulation procedure;and apply a modified stimulation current from the current source to thenerve fiber through the set of electrodes during a second stimulationphase of the neural stimulation procedure, wherein the modifiedstimulation current is generated based on a voltage difference between(i) a voltage at a first electrode in the set of electrodes, and (ii) areference voltage derived from voltages at the first electrode and asecond electrode in the set of electrodes.
 20. The neural stimulationapparatus of claim 19, wherein the stimulation circuit is furtherconfigured to adjust one of more characteristics of the stimulationcurrent to generate the modified stimulation current by: generating asignal based on the voltage difference; and supplying the signal to thecurrent source, the current source being configured to receive thesignal and adjust one or more characteristics of the stimulation currentbased on the signal.